A pile of leaves and sliced vegetables is just a pile of leaves and sliced vegetables. It needs a dressing to turn it into a salad.
A little bit of oil in our food is needed to absorb certain vitamins. Please note that I said a little bit, great slabs of greasy food are still unhealthy. As more vitamins become available the salad tastes better.
Many salads are covered with a thin dressing that is made with a base of 2 parts olive oil and 1 part vinegar or lemon juice. Then the dressing is flavoured with salt/herbs/spices to taste.
Do not use pomace oil in a salad dressing. After olive oil is extracted from crushed olives, there is still a little bit of oil remaining in the pulp. (It would make good animal feed.) However many companies use heat and chemicals to extract the remaining oil. That remaining oil cannot be called olive oil, but is called pomace or olive-pomace. While it is a poor choice for a salad dressing, pomace can be used to make soap and similar non-foods.
Some salad dressings are based on mayonnaise. The word mayonnaise was imported into the English language from France about 200 years ago, along with the food. Thousand island salad dressing is one of those made from mayonnaise. Mayonnaise is an emulsion. An emulsion is what is formed when oil and water are caused to mix. Normally oil and water are well known for not mixing. Pour the two together and the oil sits on the top. However certain things, like egg yolk, can allow them to mix. The oil mixes with the egg yolk and the egg yolk mixes with the water.
Fruit salads include chopped fruit in a fruit dressing or even in gelatin. Fruit dressings are often made with sugar balanced with vinegar or lemon juice. These dressings can also include a little oil or salt.
The base of a salad is lettuce with a little dressing. However there are many types of lettuce to choose from. Even wet markets in Hong Kong now have a few types of lettuce. Then other types of raw vegetables are chopped up and added. Common ones include tomatoes, celery, carrots, cucumber, zucchini, onion, cabbage, cauliflower, spinach, peppers and so on. Some cooked and cooled vegetables are also added, these include corn, beets, parsnips, turnip and so on.
To summarise, a salad is a cold vegetable mixture with a dressing. Without a dressing, it is not a salad, just a mixture of cold chopped vegetables.